CD8 and CD4 are cell surface proteins expressed by mature T lymphocytes that recognize or are restricted by class I or class II major histocompatibility complex (MHC) proteins, respectively, and they play critical roles in T cell activation and differentiation. Cells expressing CD8 are mostly cytotoxic in function and are important for immune responses against viruses and tumors. CD4 cells are mostly helper/inducer in function; they provide signals for B cells to generate antibodies and serve a major regulatory role for immune responses. The overall goal of this project is to further define the parameters responsible for commitment of developing thymocytes to either the CD8 or the CD4 lineage. The first specific aim is to further define the region(s) of a CD8/CD4 hybrid (CD8alpha external, CD4 transmembrane, CD4 intracellular) transgene responsible for driving cells expressing an MHC class I- specific T cell receptor (TCR) to the CD4 lineage. This will be accomplished by generation of a series of transgenic mice expressing new CD8/CD4 hybrids and mutants. Such mice will be mated to mice expressing an MHC class I-restricted TCR transgene and the effects upon T lineage commitment of cells expressing this TCR will be examined. The second specific aim is to further determine the mechanism for low numbers of TCR transgene-expressing cells seen when the TCR and CD8/CD4 transgenes are both expressed in homozygous H-2b mice and determine whether the CD8/CD4 hybrid transgene provides increased signaling for negative selection. This will be done by generating H-2bxk heterozygous double transgenics to reduce the level by selecting class I protein by 50%. The third specific aim is to determine whether the ability of the CD8/CD4 hybrid transgene to change lineage commitment extends to other class I-specific TCRs by crossing the CD8/CD4 transgenics with mice expressing different class I-specific TCR transgenes. The double transgenic progeny will be analyzed for T cell and thymocyte subsets. The final goal is to determine the functional program of the CD4 cells bearing the class I-restricted TCR transgene in mice expressing both the hybrid CD8/CD4 and the TCR transgene by examining whether these cells have a T helper cell phenotype.